Added a few checks in the agent scripts to prevent bogus information displayed by check_mk and rendered by observium

The goal is to clear bogus messages from check_mk when a user copies all of the agent files on the server. For example - the message <<<hddtemp>>> should appear only if hddtemp is installed and working.

http://dev.jokegurus.com/2012112801.tar
I'm willing to make a better documentation of the scripts and put it in wiki.

001-hddtemp - check if hddtemp binary exists & check if port is open
002-powerdns - check if binary exists
003-powerdns-recursor - check if binary exists
004-mysql - remove "echo" if mysql is not configured.
005-memcached - check if module exists
006-dmi - check if binary exists, also check if /dev/mem can be opened (on openvz it cannot be opened and errors badly)
007-unbound - check if binary exists
008-drbd - changed perl path to env perl